Lab 33 – Endothermic reaction
Instructor presentation
This lab has two components. First, the instructor demonstrates the hydration reaction with a beaker allowing a small amount of water between a beaker and wood block to freeze (see video).
Chemical equestion: NH4Cl+H2O→NH4OH+HCl
Review the SDS sheet for Ammonium chloride (sal ammoniac) NH4Cl
Note these key SDS sections for this chemical
(4) Fresh air; wash soap and water; rinse/flush eye 15-20 minutes; rinse mouth on vomit & medical assistance.
(7) Wash hands after handling
(8) Skin and eyes
(10) Reacts explosively or violently with certain compounds
(13) Disposal considerations: Dilute with water and flush to sewer.
Materials
Gather materials:
Beaker (80ml)
Glass stir sticks
Wash bottle filled with distilled water
Ammonium chloride
Small wooden block
One or several scoopulas (stainless steel recommended by plastic is okay)
Small medium large test tubes and rack (about 20ml volume) – one per student.
Lab gloves
Eye protection
Long sleeves
Lab Part One – Instructor Performs, Students Observe
Introduce the session confirming all students are adhering to safety proceedures such as no horse-play and wearing PPE. Show a copy of the SDS sheet and review sections 7, 8, 10, and 13.
Point out that the chemicals being used are ammonium cloride and water. Describe the expression “anhydrous” for ammonium choride – anhydrous.
Place all of the instructor material in clear view of students. Place these on a stable lab desk or demonstration area.
Challenge students to guess/predict how the instructor will be able to lift the wooden block without touching the block with their hands.
Moisten the wooden block with distilled water about the more than size of a quarter dollar but less than the area of the bottom of the beaker. Place the beaker on top of the wet surface. Fill the beaker with about 20ml of water.
Sprinkle the white amonium choride into the water (approximately enough to cover the bottom surface of the beaker). Stir the solution long enough for the solution to freeze the beaker to the wooden block.
Lift the beaker – thereby lifting the wooden block.
Ask the students how they think the result may have happened.
Acknowledge the explanations without agreeing or disagreeing.
Lab Part Two – Students Perform
Hand each student one test tube.
Select one student to demonstrate first. Confirm long sleeves or lab coat, protective eyewear, and gloves.
Allow student to scoop small amount of Ammonium Chloride into a test tube. Do this over a “pad” or similar item to catch the substance if it some of it misses the beaker opening.
Instruct the student to remain silent about any observations they make. (This is so all students get to have the experience without certainty what takes place).
Instruct the student to hold the test tube in one hand and fill the tube about half-way up with distilled water from the wash bottle. They can use a glass stirring rod to mix the solution. When an observation has been made, have the student carefully place their tube in the rack.
Repeat the sequence with each student.
Ask the question regarding the block and water again.
Acknowledge the responses and clarify that the water froze to attach the beaker and block together.
Introduce the additonal science concept of tensile strength of ice. Likely, the ice forms a “viscoelatic adhesion” to the glass but a mechanical form of adhesion with the wooden block (i.e., water in the pores of the wood freezes).
Ice has a tensile strength of 0.7–3.1 MPa (megapascals – a unit of pressure). This is about 1 million pacals with is also 1 million Newtons per square meter.
(0:56) Dissolving NH4Cl: An Endothermic Process
Simple procedure
- In a test tube, place a small amount of NH4Cl.
- Allow students to touch the tube to confirm ambient conditions.
- Add a small amount of distilled water.
- The reaction is endothermic and happens very quick.
- Allow students to touch the resulting (cold) test tube glass.
- Ammonium Chloride (purchase)
SDS sheet
Additional Learning Opportunity with this Lab
Why does the ice “stick” to the bottom of the beaker (glass)?
… at a chemical level, liquid water may be able spread out better on some surfaces than others. For example, water spreads very well on clean glass, while some surfaces are hydrophobic, such as Teflon.
A recent study has shown that the more water in its liquid state spreads over the surface of a solid, the more ice will adhere to this solid. Conversely, a surface with little affinity for liquid water will also have little adhesion for ice.
Why this relationship between the spread of water and the adhesion of ice? First, for ice to adhere to a cold solid, water in its liquid state must have been able to freeze on contact with the solid. Here’s a simple experiment that anyone can do:
1. Place a metal plate in the freezer or in your ice cube tray.
2. Take an ice cube and place it on the plate without taking the whole thing out of the freezer: it won’t stick.
3. Take another ice cube and let it melt slightly at room temperature (by taking it out of the freezer for a few seconds, for example), then place it on the cold plate. This time it sticks!